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United States Patent |
5,198,791
|
Shibayama
,   et al.
|
March 30, 1993
|
Surge absorber
Abstract
The surge absorber having a surge absorbing element, and first and second
wire means for electrically connecting the surge absorbing element across
the input lines of an electronic device. The first and second wire means
are connected to the surge absorber by conductive heat releasable means,
for example, a low melting point solder. The second wire means includes a
spring loaded member such that, on release of the first or second wire
means by the first or second heat releasable means, respectively, by
melting of the solder due to heat generated by the surge absorbing
element, the surge absorbing element moves away from the first or second
wire means. The surge absorber prevents an abnormal heating of the surge
absorbing element when continuous overvoltages or overcurrents pass
therethrough.
Inventors:
|
Shibayama; Takashi (Saitama, JP);
Arai; Kazuyuki (Saitama, JP);
Ikeda; Fujio (Saitama, JP)
|
Assignee:
|
Mitsubishi Materials Corporation (Tokyo, JP)
|
Appl. No.:
|
829420 |
Filed:
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February 3, 1992 |
Foreign Application Priority Data
Current U.S. Class: |
337/31; 337/15; 337/163; 361/124 |
Intern'l Class: |
H01H 083/10; H02H 001/00 |
Field of Search: |
337/14-20,28-34,1-4,163-165
361/54-59,124-129,104-111
|
References Cited
U.S. Patent Documents
3735312 | May., 1973 | Nagel | 337/15.
|
4288833 | Sep., 1981 | Howell | 361/24.
|
Primary Examiner: Broome; Harold
Attorney, Agent or Firm: McAulay Fisher Nissen Goldberg & Kiel
Claims
What is claimed is:
1. A surge absorber comprising:
a) a surge absorbing element;
b) first and second wire means for electrically connecting the surge
absorbing element across the input lines of an electronic device, the
surge absorbing element being serially connected between the first and
second wire means by first and second conductive heat releasable means,
respectively;
the second wire means comprising a spring loaded member;
wherein on release of the first or second wire means by the first or second
heat releasable means, the connection between the surge absorbing element
and the input lines is disrupted; and
wherein the assembly of the surge absorbing element and the first and
second wire means is enclosed in a housing of an insulating material, and
the first and second wire means extend exterior of the housing to provide
connection therefore with the input lines of the electronic device.
2. The surge absorber of claim 1 wherein the housing has means for
releasably securing the surge absorber to a base plate.
3. The surge absorber of claim 2 wherein the housing is a cylindrical tube
having endcaps thereon, the endcaps being adaptable for releasable
securement to clip means of a base plate.
4. The surge absorber of claim 3 wherein the tue is made from glass, the
tube and the end caps form a gas tight enclosure for the assembly, and the
interior of the housing is filled with an inert gas.
Description
BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates to a surge absorber suitable for protecting
electronic devices used for communication equipment, such as, facsimiles,
telephone switchboards, modems, and the like, from surge voltages and
continuous overvoltages or overcurrents. More particularly, it relates to
a surge absorber which includes a surge absorbing element used for
protecting the electronic devices from surge voltages and wire means for
preventing abnormal and deleterious heating of the surge absorbing element
when continuous overvoltages or overcurrents flow to the surge absorber.
II. Description of the Related Art
In the prior art, a conventional surge absorbing element, e.g., a gas
charge tube, is parallel connected to an electronic device to be protected
via a pair of input lines of the electronic device, and is designed to
operate at a higher voltage than the operating voltage of the electronic
device. Such a prior art surge absorbing element is a resistor having a
high resistance when the voltage applied thereto is lower than the
discharge voltage thereof and a resistance tens of ohms lower when the
voltage applied thereto is higher than the discharge starting voltage
thereof. Accordingly, when surge voltages, such as, lightning surges,
etc., are instantaneously applied to an electronic circuit including the
surge absorbing element and the electronic device, the surge absorbing
element discharges to suppress the surge voltages, and serves to protect
the electronic device from the surge voltages. However, when an
overvoltage or overcurrent due to an accident, etc., is continuously
applied to the electronic circuit, a certain amount of current
continuously flows through the surge absorbing element. This results in
the surge absorbing element being heated to high temperatures. The heat
radiating from the surge absorbing element can cause the protected
electronic device, as well as other electronic devices surrounding the
surge absorbing element to catch fire.
A typical example would be an accident wherein the input lines of the
electronic device contact the power lines thereof. While it does not
usually happen that such overvoltages or overcurrents resulting from such
accidents are continuously applied to the surge absorbing element, to
achieve maximum safety, it has recently become desirable to take
additional safety measures to avoid such accidental problems and fires
caused thereby. An example of such maximum safety measures are those
prescribed by Underwriter's Laboratories, Inc. of the U.S.A (UL) which
requires a safety standard for surge absorbing elements so that they do
not cause fire or electrical shock in communication equipment surrounding
the surge absorbing element when continuous overvoltages or overcurrents
are applied.
Japanese laid open patent application No. S63-18923 discloses a surge
absorber which passes these safety standards. The disclosed surge absorber
can prevent the abnormal heating of a surge absorbing element due to
continuous overvoltages or overcurrents and thereby prevent electronic
devices located close to the surge absorber from catching fire. This prior
art surge absorber disclosed in this patent application is depicted in
FIG. 4 and is composed of a surge absorbing element and a metal wire of a
low melting point metal adhered to a surface of the surge absorbing
element. The wire is electrically series connected to the surge absorbing
element. As shown, surge absorber 40 has two lead pins 42 and 43 which
pierce through an insulating base plate 41 of flameproof resin, e.g.,
polybutylene terephthalate. One end of lead pin 42 is welded to one end of
a low melting point 0.25 mm diameter metal wire 49. One end of lead pin 43
is welded to a lead wire 17 of a surge absorbing element 14. The other end
of the low melting point metal wire 49 is soldered to a lead wire 16 of
surge absorbing element 14 by solder 28. Cylindrical glass 50 having an
outer diameter of 5 mm, an inner diameter of 3.5 mm and a length of 10 mm
is attached to base plate 41 to encase the surge absorbing element 14 and
the low melting point metal wire 49. A casing or housing 45 of the same
material as base plate 41 is further attached to the base plate 41. The
respective other ends of the two lead pins 42 and 43 are inserted through
holes 47 and 48 of a print circuit board 46 and then are soldered to the
print circuit board 46.
A silver-lead (silver 7 wt.%) wire is used as low melting point metal wire
49. When a continuous overvoltage or overcurrent flows to this surge
absorber, the low melting point metal wire 49 melts (blows) from the
abnormal heating of surge absorbing element 14. This cuts the continuous
current from flowing to the surge absorbing element and then the abnormal
heating stops.
However, a problem with the above surge absorber is that the surge absorber
must be changed for a new one due to the melting of the solder connecting
the lead pins to the print circuit board after the low melting point
solder blows and changing the surge absorber is troublesome. Another
problem with this surge absorber is that it requires a sufficient amount
of space to locate the low melting point metal wire close enough to the
surge absorbing element so as to assure that it rapidly blows, and also
needs a relatively wide space between the surge absorbing element and the
inner wall of the casing to prevent a thermal modification of the casing
due to the abnormal heating of the surge absorbing element. As a result,
it is difficult to miniaturize the surge absorber.
SUMMARY OF THE INVENTION
An object of this invention is to provide a miniaturized surge absorber
comprising a surge absorbing element used for suppressing instantaneous
surge voltages and devices used for preventing an abnormal heating of the
surge absorbing element when it is subjected to continuous overvoltages or
overcurrent and thereby preventing fire or thermal damage to electronic
devices surrounding the surge absorber.
It is a further object of the invention to provide a surge absorber which
is easily removable and/or replaceable from a circuit board to which it is
normally attached so that it can easily be exchanged for a new one after
the surge absorber becomes non-functional due to blowing.
These objects are achieved by the inventive surge absorber which comprises
a surge absorbing element serially connected between first and second wire
means for electrically connecting the surge absorbing element across the
input lines of an electronic device. The first and second wire means are
connected to the surge absorber by conductive heat releasable means, for
example, a low melting point solder. The second wire means includes a
spring loaded member which, on release of the first wire member by the
first heat releasable means, e.g., by melting of the solder due to heat
generated by the surge absorbing element, urges the surge absorbing
element away from the first wire means. Thus, when heat is generated by
the surge absorber, for example, when a relatively large current at an
overvoltage flows to the surge absorber, the first wire means is blown by
the heat. Also, when a relatively small current at an overvoltage flows to
the surge absorber, the heat releasable means connecting the first wire
means melts due to the abnormal heating of the surge absorbing element.
The surge absorbing element immediately moves away from the first wire
means due to the spring elasticity of the spring loaded member.
Similarly, if instead, the second heat releasable means releases the second
wire means, the spring-loaded member is released and restored to a
non-spring loaded state. This also serves to disconnect the surge
absorbing element from the circuit.
The inventive surge absorber can be encased in a housing, usually, an
insulating housing, having connecting means for releasably attaching the
housing to a base or circuit board. This allows for easy removal of the
surge absorber after it has become disabled due being blown from
overheating as well as easy replacement with a new surge absorber.
In the present specification, the term an "overvoltage" or "overcurrent"
shall mean an abnormal voltage above the discharge starting voltage of a
surge absorbing element or a current accompanied by the abnormal voltage.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view of an embodiment of the invention.
FIG. 2 is a perspective view of the embodiment shown in FIG. 1.
FIG. 3 is an electric circuit diagram including the embodiment shown in
FIG. 1.
FIG. 4 is a sectional view of a prior art surge absorber.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Surge absorbing element suitable for use in this invention include microgap
type surge absorbers and air gap type surge absorbers. A typical microgap
type surge absorber is disclosed in U.S. patent application Ser. No.
07/798,531. Conductive heat releasable means suitable for use in the
invention include metals having a low melting point, e.g., eutectic
solders and other solders having low melting points. The wire means may
include fine phosphoric bronze wires and other fine metal wires having low
melting points.
Referring to FIGS. 1 and 2, a sectional view and perspective view of a
surge absorber of this invention are shown, respectively.
A surge absorbing element 14 used in the depicted surge absorber 13 has two
lead wires 16 and 17. It is a microgap type surge absorber having a
discharge starting voltage of 300 volts. It is manufactured by forming
microgaps 19 having a width of several tens of microns which are
perpendicular to a columnar ceramic element 18, the surface of which is
coated with a conductive thin film. Two cap electrodes, 21 and 22, are
attached to both ends of the columnar ceramic element 18. The assembly of
columnar ceramic element 18 and cap electrodes 21 and 22 is encased within
a glass tube 26 and held in place by a pair of metal endplugs 23 and 24
which serve to provide a gas-tight seal at each end of glass tube 26. The
tube is filled with an inert gas and two lead wires 16 and 17 are welded
to the respective ends of metal plugs 23 and 24.
The surge absorber is manufactured by the following steps:
1. Lead wire 16 is soldered to one end of a fine phosphoric bronze wire 27
having a diameter of 0.15 mm and melting point of 910.degree. C. by a
eutectic solder having a melting point of 183.degree. C. The lead wire 17
is soldered to one end of a coil spring 29 of a phosphoric bronze wire
having a diameter of 0.3 mm by a eutectic solder 30 having a melting point
of 183.degree. C.;
2. The fine metal wire 27, the surge absorbing element 14 and the coil
spring 29 are positioned linearly and inserted into a lead glass tube 31
having a diameter of 5 mm and a length of 20 mm;
3. The other end of the coil spring 29 is soldered to a center hole 33a of
a metal cap 33 by a eutectic solder having a melting point of 183.degree.
C. The free end of fine metal wire 27 is placed through a center hole 32a
of metal cap 32, and both ends of the glass tube 31 are inserted into the
metal caps 32 and 33;
4. The free end of fine metal wire 27 which traverses center hole 32a is
pulled to stretch coil spring 29 and place it, and, in turn, wire 27 and
surge absorbing element under spring load. The free end of metal wire is
then soldered to the center hole 32a with a eutectic solder having a
melting point of 183.degree. C.
Metal caps 32 and 33, make it easy to put in or take off the inventive
surge protector from clip holders 34 and 35 of a base plate 36.
FIG. 3 is a circuit diagram of the surge absorber shown in FIGS. 1 and 2.
As shown, the surge absorber 13 is parallel connected to, i.e., across, an
electronic device 10 to be protected through a pair of input lines 11 and
12. A fuse 15 is series connected between a part of the input line 11 and
the cap holder 34 to prevent continuous overvoltages or overcurrents from
being applied to the electronic device 10.
A fine electric current at an overvoltage, i.e, an electric current of 0.25
amperes at AC 600 volts, is passed to input lines 11 and 12. After 4
seconds from starting the current flowing, solder 28 is blown (melted) by
the heat generated by surge absorbing element 14 and the current flow to
surge absorber element 14 is stopped. As a damage.
In another example, a small electric current at an overvoltage, i.e, an
electric current of 2.2 A at AC 600 volts, is introduced to input lines
11 and 12 of the circuit shown in FIG. 3. After 2 seconds from starting
the current flowing, solder 28 is blown by the heat generated by the surge
absorbing element 14 and the current flowing to the surge absorber element
14 is cut off. As a result, electronic device 10 does not suffer any
thermal damage.
In each of the above cases, the surge absorber having the blown solder
connection can be easily removed from the cap holders 34 and 35, and a new
surge absorber can be easily attached thereto. Also, because of the
linearity, the inventive surge absorber 13 has a relatively small size,
i.e., a diameter of 5 mm and a length of approximately 20 mm, as compared
to that of prior art surge absorbers. Thus, it can be used to mount
various devices on a print circuit board in close proximity to one
another.
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